Shaft encoder



Dec. 16, 1969 D. H. MARGOLIEN ET AL 3,484,776

SHAFT ENCODER Filed Jan. 4, 1965 2 Sheets-Sheet 2 United States PatentUS. Cl. 340-347 12 Claims ABSTRACT OF THE DISCLOSURE An improvedrotational shaft encoder comprising a novel mounting block having aplurality of cavities and having a plurality of elongated pin-likecontact elements disposed Within the cavities to move in a directionparallel to the longitudinal axis of the contact element. Each contactelement has a flange which is closely spaced from the cavity wall toprevent transverse movements of the contact element, and in addition theflange cooperates with the rotating commutator to provide a pumpingaction to lubricate the point of contact between the contact element andthe commutator and to clear away wear particles from the commutator.Additionally, a novel commutator disc is disclosed having selectedtracks divided into displaced Zones so as to allow redundancy andconsiderable contact placement flexibility.

The present invention relates in general to an improvedanalog-to-digital converter and in particular to a rotational shaftencoder employing a novel contact element (commonly called a brush) andmounting block arrangement for converting shaft position into a digitalnumber representative of the shaft position.

As is well known in the prior art, an analog signal in the form of ashaft rotation may be converted to a digital signal by means of arotational shaft encoder. Basically, these encoders include a rotatableencoder disc, commonly called a commutator, having a separate annulartrack or zone for each binary digit of the digital number to berepresented. Each annular track includes separate segments or areaswhich are representative of the value of the binary digit represented bythe particular annular track. In the majority of encoders, the areascomprising the annular rings are alternately electrically conductive andnon-conductive. When an electrically conductive contact brush is placedin contact with one of the annular rings on the commutator, anelectrical current flows through the brush whenever the brush contacts aconductive area. In this manner, the value of each digit of the binarynumber which is representative of the shaft position is determined. Byconnecting an electrical conductor to each of the brushes, the signalrepresenting the binary number can be externally applied to an automaticdigital computer. It should be noted at this point that while there areshaft encoders on the market of the non-contact variety, such asmagnetic encoders or optical encoders, still the brush-type shaftencoder is the simplest, the most reliable, and the least expensive.

It has been found, however, that prior brush encoders suffer fromseveral limitations. These limitations are caused mainly by the type ofL-shaped spring contact element which is used in most of the presentcommercial encoders. During the operation of these commercial encoders,the commutator often rotates at relatively high speeds. Because of thepresence of dust or wear particles and any unevenness of the commutatorsurface, the spring contacts tend to bounce up and down and to have sideto side motion. These oscillations and motions sometimes progress intodestructive resonant oscillations which cause extremely large G forcesto be applied to the commutator surface and cause the points and edgesof the contacts to dig into the surface of the commutator. In thecommercial contact encoder, the bouncing of such contacts is generallycounteracted by holding the contacts against the surface of thecommutator with a relatively large amount of force. While in some casesthis is partially successful, this large amount of force causes anexcessive amount of friction to be generated at the surface of thecommutator and results inevitably in a substantial wearing of both thesurface of the commutator and the contact element itself, greatlyshortens the life of the encoder and creates a significant amount ofnoise. It is apparent, moreover, that once wear particles have beencreated on the surface of the commutator, an irreversible process ofdegradation of the encoder performance takes place. The wear particlescreated on the surface of the commutator cause the contact elements tobounce even more violently and also contribute to sideways flutter andchatter. In addition, the now roughened surface causes even a greateramount of friction to be generated, thereby distorting the position ofthe L-shaped spring contact element along its annular track. It isapparent, of course, that once being generated these wear particlesremain on the surface of the commutator, the metal particles causing thedevice to short out and the insulating particles causing the device tohave open circuits. In addition, the bouncing of the contact elementscauses the encoder to miss counts and generally cease to functionadequately. It should also be noted that once the contact elements havebeen affixed to the brush blocks of commercial encoders, there is nosimple or economical method for adjusting the position or pressure ofthe contacts with respect to the commutator. It is all too obvious thatany initial inaccuracies or subsequent changes in contact pressure andcontact position are detrimental to the proper performance of aprecision encoder.

Some attempts have been made in the encoder field to overcome the poorperformance of encoders by providing more than one contact element foreach annular track, thus hoping to obtain accuracy through redundancy.If these multiple contact elements are placed inline, however, theabove-stated disadvantages resulting from the excessive wear received bythe single track on which all of them are riding far exceeds anyadvantages obtained by the redundancy feature. In many types ofencoders, it has been attempted to place the contact elements side byside. This has resulted, however, in the fabrication of a commutatorwhich is larger or has less digit tracks or in the fabrication of finerand more fragile contacts. It is apparent that the larger commutator orthe commutator with less digit tracks make the encoder less inherentlyuseful. The finer contact elements, on the other hand, are subject tofaster wear, more violent bouncing because of their lesser weight andmore deflection and flutter because of their thinner structure; inaddition, they are less able to endure contact pressure. The combinationof these disadvantages far outweigh any gains which may be obtained fromtheir redundancy features.

The present invention has succeeded in overcoming all of thedisadvantages of the prior art devices by providing a triply redundantcontact encoder in which a plurality of large, well formed contactelements accurately maintain a precisely predetermined position withrespect to the code pattern of the commutator. The mounting block of theencoder is designed to allow each contact element to be mechanicallyindependent, to be individually spring loaded against the commutator,and to have an extremely small axial wobble. The present encoder is alsodesigned to have an oil lubrication, flushing and damping system inwhich the structure of the contact elements and the mounting block, inconjunction with various adhesion and Bernoulli forces, serves to dampthe motion of the contact and, by pumping action, to reciprocallycirculate oil along the surface of the contact elements and thecommutator to lubricate the motion of the contact element over thecommutator and to flush or remove wear particles from the surface of thecommutator and deposit them in an oil cavity within the mounting block.The provision for redundancy in the present invention has been combinedwith a novel commutator arrangement which, instead of requiring theredundant contact elements to be diminished in size and placed closelytogether, enables the contact elements to be large and well formed andto be widely spaced apart. This novel commutator arrangement enables thecontact elements to be placed so as to optimize design features withoutincreasing the fabrication difficulty and expense ordinarily attendantin the use of a large number of contract elements.

It is, therefore, the primary object of the present invention to providea new and improved rotational shaft encoder of the electrical contactvariety.

It is another object of the invention to provide an encoder in whichboth the location and the contact pressure of the contact elements canbe easily and accurately predetermined.

It is a further object of the present invention to provide an encoder inwhich the contact elements suffer from minimal deflection, contactbounce and friction.

It is still another object of the present invention to provide anencoder in which the contact element surface is continuously lubricatedand any wear particles are swept away from the contact region anddeposited in the mounting block.

It is a still further object of the present invention to provide anencoder in which no final adjustment of the location or contact pressureof the contact elements is necessary.

It is still another object of the present invention to pro vide a highlyaccurate and reliable encoder having a long life, capable of easyfabrication and inexpensive to manufacture.

It is a further object of the present invention to provide a novelcommutator.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings. It is to be expressly understood, however, thatthe drawings are for the purpose of illustration and description onlyand are not intended as a definition of the limits of the invention.

FIGURE 1 is a simplified cross-section view of a rotational shaftencoder containing the present invention;

FIGURE 2a is an isometric view of the mounting block assembly of thepresent invention;

FIGURE 2b is a cross-sectional view of the mounting block assembly ofFIGURE 2a;

FIGURE 20 is a detailed cross-sectional view of the pin-like contactelement of the present invention;

FIGURE 3 illustrates the novel commutator employed in the presentinvention; and

FIGURE 4 illustrates waveforms obtained from a connected set of contactelements.

In the description of the invention to follow, corresponding referencednumerals have been carried forward throughout the figures to designatelike parts of the invention.

In FIGURE 1 there is shown a rotational shaft encoder generallydesignated as 10. The encoder includes a housing 12 and a support member14 inserted therein which holds an input shaft 16, a pair of bearings 18and a code disc or a commutator 20. The commutator 20 is con entricallyco pled to the shaft 16 for rotation there- 4 with. A mounting block 22is screwed to the housing 12 and has a plurality of contact elements 24FIGS. 2a and b positioned therein and held in tension against thecommutator 20. Leads 26 connect the contacts 24 to a plurality ofterminals 28. The terminals 28 are connected to a diode package 30 whichcontains a plurality of blocking diodes coupled to the leads 26 toisolate the encoder 10 from erroneous external signals. The leads 26 arecoupled through the diodes of diode package 30 to a correspondingplurality of external coupling connectors 32 which extend out of therear of encoder 10. The external coupling conductors 32 and the diodepackage 30 are held by support 34 which is screwed into the housing 12of the encoder 10.

The mounting block 22 is shown in more detail in FIG- URES 2a, b, and c.The mounting block 22 is composed of a lower block part 22a and an upperblock part 22b. With each of the block parts 22a, 22b there is aplurality of precisely positioned cavities; each of the block parts alsoform shoulders 36a, 3612 respectively (formed, for example, bycounterboring). When the block parts 22a, 221) are placed together inregistration, a plurality of cavities 23 having retaining shoulders 36a,36b are formed in the block 22 in which the contacts 24 are contained. Aplurality of slots 27 are also formed in the lower block part 22a intowhich the U-shaped terminals 28 are inserted. Preselected ones of thecontact elements 24 are connected by leads 26 to form a singleelectrical contact. The exact placement of the contact elements 24 inthe brush block 22 and their electrical intercouplings will be discussedin more detail in connection with FIG- URE 3.

In FIGURES 2a, b, and c, the contact elements 24 are shown as having anelongated, pin-like structure with a flange 38 near one end thereof. Thecontact element 24 is constrained by the cavity 23, the flange 38 andthe shoulders 36a, 36b to have limited motion in the direction of thecommutator 20 and to have minimal or transverse motion (deflection) andaxial wobble. In the present device, the spacing between the shouldersand the contact is maintained between .2 and .3 mil; a larger spacing of11.5 mils is maintained between the flange 38 and the walls of thecavity 23. The contact element 24 is forced towards shoulders 36a by aspring 40 which encircles the contact element 24 and is compressedbetween shoulder 36b and flange 38. The spring 40 provides an elasticrestraining force for any bouncing motion of the contact element 24normal to the commutator 20 and a very accurate contact pressure(approximately 1 gram) on the surface of the commutator 20. The lengthof the contact element used in this embodiment is approximately 100mils, the flange diameter, 30 mils, the center shaft diameter, 20 milsand the end shaft diameters 15 mils. The contact is composed ofapproximately 20% Cu and Au and has a hardness of approximately 300-350Knoop. The commutator 20 may be composed of a plastic material, such asepoxy, while the conductive areas thereof, such as area 42, may becomposed of Au plated on a Ni-Fe base.

In the invention, the cavities 23 are filled with a lubricating fluid,such as oil. The fluid is initially put on the commutator 20 (separatedfrom lower block part 22a by approximately 5-6 mils) which is thenrotated with respect to the block 22. It is believed that the motion ofthe contact element 24 with respect to the fluid causes (according toBernoullis theorem) a stagnation pressure greater than the generalpressure in the fluid to exist in front of a contact element 24. Thispressure coupled with adhesion forces (causing capillary action) inducesthe fluid to flow up the contact element end into the cavity 23 and outonto the upper block part 221); when the fluid appears on upper blockpart 22b, cavity 23 is completely filled. It has been experimentallydetermined that the placing of the fluid in cavity 23 greatly improvesthe performance of the device. First, the fluid effectively (lamps anyoscillatory motion of contact element 24 and thus reduces to a minimumany bouncing action (and missed counts). Secondly, the fluid provides afilm of lubrication between contact element 24 and the surface of thecom- Inutator 20. This film reduces friction to a minimum and allows alarge gram pressure to be put by contact element 24 on the commutator 20to further reduce bouncing action. Whatever bouncing action remains isutilized to pump fluid down the shaft of the contact element 24 and inconjunction with the above-mentioned forces to form a recirculatingsystem. In addition, any particles of epoxy or metal that are on thecommutator 20 are either swept out of the path of the contact element 24or are caught up by this recirculating system and are deposited in thecavity 23. Thus, the fluid filled cavity 23 acts as a lubricationreservoir to reduce friction, remove unwanted particles from the path ofthe contact element 24, and provide hydraulic damping for the contactelement 24 to minimize contact element bounce.

In FIGURE 3 the novel commutator employed in the present invention isillustrated. The commutator 20 comprises a series of annular designatedtracks through 8. As explained more fully on pp. 6-40 through 6-49 ofNotes on Analog-Digital Conversion Techniques, edited by Alfred K.Susskind, copyrighted 1957, the MIT Press, Cambridge, Mass, track 0 iscalled the least significant track and is comprised of a series ofalternating conductive and nonconductive segments 50 and 52. Inaccordance with the 2 progression of the binary code track 1 is composedof a series of alternating conductive and nonconductive segments 56 and54 whose width is twice that of the segments in track 1. In a similarfashion, track 2 has segments twice the width of those in track 1, andtrack 3 twice the width of those in track 2. In the novel commutator ofthe present invention, however, the three zones 4a, 4b and 40 comprise asingle track whose segments have a width twice that of those in track 3.By splitting this single track into a plurality of concentric annuli oflike significance whose corresponding points or binary patterns havebeen circumferentially displaced from one another, it is now possible(as explained more fully hereafter) for a considerable number of large,wellformed and well-spaced contact elements to be set in the mountingblock in positions which minimize difficulties in manufacture, assemblyand maintenance. In a similar fashion, zones 50, 5b and 5c comprise asingle track and zones 6a, 6b and 6c comprise a single track. Track 7 isis termed the most significant track and track 8 acts as an electricalcommon for all the preceding tracks.

In'this embodiment of the invention, the V-scan reading method isemployed, which method is fully described in the aforementionedreference. In brief, this method requires that a single contact elementbe placed on the least significant track, its position defining areading index line, and a pair of contact elements spaced an appropriatelead and lag distance from the reading index line be placed on all othertracks (except the common) to ensure correct logic readout. It should benoted however, that while in theory the contact elements in the V-scanmethod of reading form a V, in practice this does not have to be thecase. Once the contact elements have been placed in proper position forV-scan logic any contact element may be shifted along its own track 2nsegments (i.e. an even number thereof) and still give the proper readingfor the V-scan logic. Moreover, each entire track may be shifted adesired amount along with its respective contact elements. In addition,as explained in the aforementioned reference on p. 6-48, each contactelement may vary iVs of a segment (on its own track) from its optimumplacement (or i% the previous track segment length). As will beexplained hereafter, this allowed variance has been employed in thepresent invention to determine whether the contact elements on selectedtracks are making proper electrical contact.

As was illustrated in FIGURE 20 of the present invention, a plurality ofelectrically coupled contact elements are used for each contact elementnecessary in the V-scan method of reading in order to give the deviceredundancy and thus greatly increase its reliability. These contactelements are represented in FIGURE 3 by a plurality of dots 58. In track0, three connected contact elements are shown positioned on segmenttransition lines, each occupying its Own separate track. In track 1, sixcontact elements are shown, three contact elements leading theirrespective transition lines by one-half the previous segment length andthree contact element lagging their respective transition lines byone-half the previous segment length. As in track 0, track 1 is dividedinto three annuli (zones) with two contact elements on each zone. Tracks2 and 3 have contact element placements essentially identical to thosein track 1 with the appropriate contact element spacings from thetransition lines (which lines are the equivalent of the reading indexline because of the contact element placement in track 0) being used, asexplained in Equation 6-20 on p. 6-48 of the aforementioned reference.

As described previously, zones 4a, b, c, 511, b, c, and 6a, b, ccomprise three tracks which are each split into three separate annulicircumferentially dispaced from one another, each annuli having twocontact elements located thereon. Although some ease in contact elementplacement has been obtained from shifting each contact element an evennumber of segments or from shifting entire tracks and their respectivecontact elements, as explained previously, the foregoing feature of thepresent invention, the internal displacement of zones of a single track,provides a degree of flexibility in contact placement unknown in priorart devices. This flexibility in contact element placement not onlyallows triple redundancy to be used in the present device withoutsacrificing desir ability and accuracy but also allows certain designfeatures to be optimized such .as the size and structure of the contactelements, the size of the commutator, the spacing betweenelectricallycoupled contact elements, the spacing between contact elements ondifferent zones and tracks, and the elimination of crossed lead wires.The significance of this feature is emphasized by the fact that fiftyindependent contact elements are positioned on a commutator .85 inch indiameter and, more particularly, on an annular section thereof having a.295 inch LD. and a .84 inch O.D. (four contact elements being on track8 and a zero reference contact element on the outer edge). In addition,no more than two contact elements ride on the same track, and with minormodifications each contact element could ride on its own separate track.

This flexibility in contact element positioning makes possible, inaddition, the determination as to whether the the contact elements onpreselected tracks are making proper electrical contact. While, asstated previously, the contact elements in an encoder are generallyplaced a preselected distance (the optimum position) from the rteadingindex line, the contacts may have 1- /8 segment tolerance from suchoptimum position. In the present invention, each set of three contactelements on tracks 4-7 has one of its members placed at the optimumposition and the other :Ms segment from the optimum position. If thesignal from an electrically coupled trio of contact elements ispresented on an oscilloscope, an examination of the duration of thewaveform and the make-break ratio (the length of time the contactelement is conducting to the length of time the contact element isnonconducting) and an examination of the position of the trailing andleading edges of the conducting waveform enables the determination as towhether any of the contact elements (except the central one alone) isnot making proper electrical contact.

This method of determination is further illustrated with reference toFIGURE 4. In FIGURE 4, a trio of contact elements labeled A, B, C areshown traveling in the direction of the arrow towards a conductiveregion. The

waveform resulting from one, two or three of the contact elements makingelectrical contact is shown in (a) through (f). In (a) the waveformshown is generated when contact elements, A, B, C, or A, C, are makingelectrical contact with the conductive region. It is thus not possibleto determine whether contact element B alone is functioning properly.If, however, contact element C alone does not make electrical contactwith the conductive region, then the leading edge of the waveform isdelayed as shown in (12). Similarly, if contact element A alone does notmake contact, then the trailing edge of the waveform arrives early asshown in (c). In a similar fashion, (d), (e), and (f) show the waveformgenerated when only contact elements A, B, C, respectively, are makingelectrical contact with the conductive region. In addition, for theparticular commutator shown, the make-break ratio when only one contactelement is making electrical contact is 50/50, when two are makingcontact, 60/40, and when three are making contact, 65/35. In such amanner, nearly complete information can be obtained on the electricalcontact characteristics of the contact elements.

Having described the invention, it is apparent that numerousmodifications and departures may be made by those skilled in the art;thus, the invention herein described is to be construed as limited onlyby the spirit and scope of the appended claims.

What is claimed is:

1. Electrical contact apparatus comprising:

a mounting having a cavity capable of containing a liquid;

an elongated electrical contact element, said contact element mountedwithin said cavity and movable therein in a direction parallel to thelongitudinal axis of said contact element;

means disposed about said contact element and closely spaced from theinterior wall of said cavity, for allowing restricted fluid passagealong said interior wall; and

a surface on said means disposed about said contact element cooperatingwith said mounting for substantially preventing movement of said contactelement in a direction transverse to the axis of said contact element.

2. The electrical contact apparatus of claim 1 including a springencircling said contact element for providing a unidirectional elasticrestraining force to axial motion thereof.

3. The electrical contact apparatus of claim 1 including a liquid withinsaid cavity for dampening the axial motion of said contact element andfor removing wear particles from and lubricating said contact element.

4. An encoder having a rotatable commutator comprising in combination:

a mounting having a cavity capable of containing a liquid;

an elongated electrical contact element, said contact element mountedwithin said cavity and movable therein in a direction parallel to thelongitudinal axis of said contact element;

means disposed about said contact element and closely spaced from theinterior wall of said cavity, for allowing restricted liquidpassage-along said interior wall; and

a surface on said means disposed about said contact element cooperatingwith said mounting for substan tially constraining movement of saidcontact element in a direction transverse to the axis of said contactelement.

5. Electrical contact apparatus as claimed in claim 1 wherein:

said means disposed about said contact element is a flange and isintegral with said contact element for moving a liquid within saidcavity into and out of said cavity.

6. Electrical contact apparatus comprising:

a block having a cavity therein;

a pin-like contact element having two ends constrained to move in saidcavity and project therefrom, said contact element including a flangewithin said cavity positioned a preselected distance from one of theends of said contact element;

a spring having two ends, a first end positioned against said flange andthe second end positioned within the cavity against the block forbiasing said contact clement; and

a liquid located in said cavity for providing dampening of the movementof said contact element and for lubricating and for removing wearparticles, said contact element and said block being spaced from eachother to allow a preselected amount of said liquid to flow between saidblock and said flange.

7. Electrical contact apparatus comprising:

a block having a cavity therein:

said block comprising first and second connectable parts, each of saidparts having a cylindrical aperture therethrough and a counterboretherein for forming a pair of shoulders, so that there is one shoulderat each end of said cavity formed when said first and second parts areconnected;

a pin-like contact element having two ends constrained to move in saidcavity and project therefrom, said contact element including a flangewithin said cavity positioned a preselected distance from one of theends of said contact element; and

a spring having two ends, a first end positioned against said flange andthe second end positioned within the cavity against the block forbiasing said contact element.

8. The electrical contact apparatus of claim 7 wherein said springencircles said contact element and is biased between one of saidshoulders and said flange of said contact element.

9. An encoder comprising:

an input shaft capable of rotation;

commutator means coupled to rotate with said shaft,

said commutator means having a code pattern thereon for generatinginformation representative of the angular position of said shaft; and

means cooperative with said commutator means for deriving saidrotational information, said mean including a plurality of independentlymounted pinlike contact elements, preselected ones of said contactelements being electrically coupled, a mounting having a plurality ofcavities capable of containing a liquid, each of said cavitiescontaining one of said contact elements in preselected positions, saidmounting for cooperating with said contact elements to limit motion ofsaid contact elements substantially in an axial direction and meanslocated within said cavities for elastically restraining the axialmovement of said contact elements.

10. The encoder of claim 9 wherein said mounting means includes withinsaid reservoir means for lubricating the contact portion of saidcontacts and for damping the motion of said contacts.

11. In an encoder having a commutator with one or more tracks thereoncomposed of discrete conducting segments, preselected ones of saidtracks being divided into a plurality of annular zones, each of saidzones being angularly displaced from one another, the combinationcomprising:

a block;

a plurality of independently mounted electrical contact elementsextending therefrom, a preselected number of said contacts beingpositioned to make electrical contact with each track and with each ofthe zones of a divided track;

said electrically coupled contact elements being positioned within2n:L/s segments from a pro-chosen 9 10 position, Where 211 indicatesdisplacement, if any, of References Cited :Edeven number of segments ofthe contacted track; UNITED STATES PATENTS means for electricallycoupling preselected ones of said 1,043,75 9 11/1912 Fishercontactelements making electrical contact with the 5 2,796,472 6/1957 Carter.plurality of Zones of a single track so as to yield a 3,030,617 4/1962Chase 340347 single output signal therefrom.

12. The combination of claim 11 wherein at least one MAYNARD R. WILBUR,primary Examiner of said electrically coupled contact elements is placedsubstantially at said pre-chosen position, at least one of 10 GARY R.EDWARDS, Assistant Examiner said coupled contact elements is placedsubstantially 2n+ /s segments therefrom, and at least one of said US.Cl. XJR. coupled contact elements is placed substantially 2/2- /s200-l66 segments therefrom.

T2233? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.7 Dated December 16, 1969 Invencor(s) David H. Margolien and Neils KragIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 3, line 19, "contract" should be --contact-- Col. 4} line 38, after"minimal" add --sideways-- Col. 6, line 24,- "dispaced' should be--displaced--" Col. 6 line 57, "rteading" should be --reading-- Col. 8,Claim 10, should read as follows:

--Claim 10. The encoder of Claim 9 including a liquid within saidcavities for lubricating the contact elements and for'dampeningthe'axial motion of said contact elments.--

SIGNED AND SEALED JUL21I970 SEAL) Attest:

Edward M. Fletcher, Ir. WILLIAM E. SOHUYLER, m. Attesting OfficerComnissioner '01 Patents

